As a digital cordless telephone system for business use, DECT (Digital Enhanced Cordless Telecommunications) technology that uses 1.9 GHz radio-frequency band has been newly standardized in Japan. Accordingly, a situation occurs where the radio-frequency bandwidth used in the PHS (Personal Handy-Phone System) digital cordless telephone system in conformity with the RCR STD-28 standard developed by the Association of Radio Industries and Businesses as the second generation personal handy phone system and the radio-frequency bandwidth used in the DECT digital cordless telephone system overlap each other.
For the radio-frequency bandwidth in the DECT standard, the approval and operations with consideration to regionality are implemented, such as 1,880 to 1,900 MHz in Europe as a standard, 1,920 to 1,930 MHz in North America, and 1,910 to 1,930 MHz in South America. On the other hand, the DECT radio-frequency band approved in Japan corresponds to five channels: the channels F1 to F5 each with a bandwidth of 1.728 MHz, existing between 1,893.5 MHz and 1,906.1 MHz, which is the same as that of the PHS radio-frequency bandwidth. The radio-frequency bandwidth used in the PHS technology corresponds to 42 channels (including control channel): the channels ch1 to ch37 each with a bandwidth of 300 kHz, existing between 1,893.5 MHz and 1,906.1 MHz and the channels 251 to 255 for the increased bandwidth of PHS RCR STD-28 Ver.3. Note that, although Japanese DECT standard is generally called “J-DECT”, it is referred to simply as “DECT” hereinbelow.
FIG. 10 is a view for explaining the communication channel allocation status in the Japanese DECT (J-DECT) standard. As shown in FIG. 10, the Japanese DECT uses five frequency bands of 1,893.5 MHz+2.116 MHz+1.728 MHz×m (m=0 to 4), which are five channels: F1 with a center frequency of 1,895.616 MHz (1,894.752˜1,896.480 MHz), F2 with a center frequency of 1,897.344 MHz (1,896.480 to 1,898.208 MHz), F3 with a center frequency of 1,899.072 MHz (1,898.208 to 1,899.936 MHz), F4 with a center frequency of 1,900.800 MHz (1,899.936 to 1,901.664 MHz), and F5 with a center frequency of 1,902.528 MHz (1,901.664 to 1,903.392 MHz).
FIG. 11 is a view for explaining the time slot structure of a standard slot in the Japanese DECT. In the DECT time slot structure shown in FIG. 11, one frame is 10 ms. The one frame is, when it is a standard slot, made up of 24 time slots where guard spaces of 56 bits (48.61 μs) each are interposed therebetween. These 24 time slots are made up of time slots S1 to S12 as time slots for transmission from a master unit to a slave unit (downlink) and time slots S13 to S24 as time slots for transmission from a slave unit to a master unit (uplink). Half-duplex communication is performed by the 24 time slots. Each time slot is composed of 32-bit Sync.-fields, 388-bit D-fields, and 4-bit Z-fields. One frame contains 11520 bits {(32+388+4+56)bits/time slots×24 time slots} of information. Because one frame is 10 ms, the information transmission rate is 1.1520 Mbps.
FIG. 12 is a view for explaining a time slot structure of a wide-band slot in the Japanese DECT, FIG. 12(A) shows the case of a Long Slot, and FIG. 12(B) shows the case of a Double Slot. As shown in FIG. 12, the time slot structure of a wide-band slot in DECT is made up of 12 time slots where guard spaces of 216 bits (187.49 μs) or 56 bits (48.61 μs) each are interposed therebetween, where time slots S1 to S6 are time slots for transmission from a master unit to a slave unit (downlink) and time slots S7 to S12 are time slots for transmission from a slave unit to a master unit (uplink), and half-duplex transmission is performed. Each time slot is composed of 32-bit Sync.-fields, 708-bit or 868-bit D-fields, and 4-bit Z-fields, and each frame of 10 ms contains 11520 bits {(32+708+4+216)bits/time slots×12 time slots or (32+868+4+56)bits/time slots×12 time slots} of information (accordingly, the information transmission rate is 1.1520 Mbps each).
FIG. 13 is a table showing a comparison between PHS and DECT both using a 1.9 GHz radio-frequency band as digital cordless telephone systems. As shown in FIG. 13, because the frequency band used is 1,893.5-1,906.1 MHz (PHS: 1,893.5 MHz+0.150 MHz+300 kHz×n (n=0˜41), DECT: 1,893.5 MHz+2.116 MHz+1.728 MHz×m (m=0˜4), which frequency bands of PHS and DECT overlap each other, interference can occur at the same frequency. Further, because the time-base frame structure of PHS is different from that of DECT (frame period: 5 ms (PHS), 10 ms (DECT), the number of multiplexed slots: 8 (PHS), 24 in normal and 12 in wide band (DECT)), interference avoidance by time sharing is not available. Thus, in the case where both of those two digital cordless telephone systems are located in communication areas overlapping each other, and the frequency bands of the used communication channels overlap each other and their timings in TDMA (Time Division Multiple Access) coincides with each other, the occurrence of a situation where interference of their radio waves occur so as to degrade the communication quality and cause communication errors is unavoidable.
Generally, in order to avoid such radio wave interference, channel switching that switches a channel under communication where radio wave interference occurs to another channel is effective as described in Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2011-77985 “Radio Communication Device and Radio Communication Method”. However, in the case of Patent Literature 1, the digital cordless telephone system that performs channel switching is radio equipment in the DECT system, not radio equipment in the PHS digital cordless telephone system for business use.